This invention relates to preparation of expanded thermoplastic articles and more particularly to a rapid-cycle and energy and time conserving process for preparing low-density molded thermoplastic articles.
In one of the most common methods of producing molded articles from expanded thermoplastic materials, thermoplastic particles such as beads or pellets containing a liquid expanding agent are heated to a temperature above the boiling point of the expanding agent and the softening temperature of the thermoplastic material. The thermoplastic material expands under the pressure exerted by the expanding agent. After the desired degree of pre-expansion is obtained the thermoplastic particles are removed from the pre-expander and permitted to cool whereby they re-assume their rigid nature but are enlarged about 20 - 40 times their original size. As the temperature of the particles drops the residual expanding agent in the particles condenses, thereby forming a partial vacuum in the hollow cells of the particles. If these particles are placed into a mold and expanded with permitting the pressure within and without the cells to reach equilibrium by gas diffusion, the expandable particles or beads will collapse upon being reheated to the softening temperature of the thermoplastic material. To avoid this it is common practice to permit the beads to age for a period of about 1 - 24 hours during which time the pressure inside the beads reaches atmospheric pressure. The beads can then be heated without danger of collapse. The term softening temperature as used above refers to the temperature at which the thermoplastic material will begin to change from a rigid to a soft state.
In manufacturing molded articles a predetermined amount of aged beads or pellets are placed in a mold and directly heated in the mold by heating up the mold. Since the thermoplastic material is an excellent heat insulator a great deal of heat must be put into the mold to insure that all of the particles reach the fusion temperature at approximately the same time, which is necessary to produce molded products having a uniform density. After the beads or pellets are fused together, the molded article must be cooled again. A large amount of cooling medium must be passed through and around the mold to cool the molded article to a temperature at which it is rigid before it can be removed from the mold. This obviously requires a relatively long molding cycle.
A recent patent, U.S. Pat. No. 3,417,170, issued to Knapp on Dec. 17, 1968 discloses a method of reducing the cycle time in a molding process for preparing expanded thermoplastic articles. In the process disclosed in this patent pre-expanded beads are pre-heated to a temperature above the boiling point of the expanding agent but slightly below the fusion temperature of the thermoplastic material prior to being introduced into a closed mold. When the heated beads are placed in the mold it is only necessary to heat them slightly to raise them above their fusion temperature. Therefore, it is not necessary to introduce great amounts of heating medium into the mold, hence the molding cycle time is greatly reduced.
Molding processes are known in which expandable thermoplastic particles are heated by means of a hot noncondensable dry gas such as heated air and then introduced directly into the mold thereby eliminating the aging and reheating steps. These processes have generally limited applications such as the molding of small objects such as cups or other small containers because of the high insulating properties of the thermoplastic material and the low heat transfer coefficient of the hot gas.
It is usually preferable to pre-expand thermoplastic expandable particles by steam, rather than hot air because steam is more efficient and conservant of energy in that it provides much greater heat transfer than hot air because steam gives up both sensible heat and latent heat of condensation. However, when steam is used in the pre-expansion the pre-expanded particles cannot normally be directly injected into the mold because they are wet and do not readily flow. The aging period, mentioned above, also serves to dry the surfaces of the beads so that when they are injected into a mold they will be free-flowing. In another patent, U.S. Pat. No. 3,129,464, issued to Heider on Apr. 21, 1964, thermoplastic expandable particles are pre-expanded by steam and are then transferred almost immediately to a mold station. It appears that although the pre-expanded particles or beads may be wet from the steam this problem is overcome by the use of plungers which force the beads first into a transfer vessel and then into the mold which is shaped to be force filled in this way. However, many molds are not shaped to be force filled and therefore dry easily flowable beads are preferred for most custom molding operations to provide easy and complete filling of variously shaped molds. Further, Heider requires further heat as well as pressure by a force plug to form the article into its desired shape which is not applicable to many expandable molding operations.
A process has now been discovered by which certain advantages have been gained over those presently known processes for molding thermoplastic expandable particles including those referred to above. More specifically the process which begins with pre-expanding of the thermoplastic particles with steam avoids the necessity of aging the particles or beads to permit pressure equilibration and at the same time provides a dry easily flowable expanded bead which can be molded in any shaped mold with greatly reduced cycle times. Shortening of the cycle time is accomplished in the portions of the cycle both before the beads are inserted into the mold and during the cooling portion thereof while the molded object is cooling in the mold. Further the present inventive process results in conservation of energy and time presently necessary in the heating phases of the molding cycle.
Accordingly, it is an object of this invention to present an improved process for molding expanded thermoplastic articles.
It is another object of this invention to present a method of reducing the molding cycle time in the manufacture of molded, expanded thermoplastic articles.
It is a further object of this invention to present a process for molding expanded thermoplastic articles from freshly pre-expanded beads, said beads having been pre-expanded by steam.
It is a still further object of this invention to present an improved process for molding expanded thermoplastic articles which result in conservation of energy and thus reduces cost.
These and other objects of the invention will become more readily apparent as the description proceeds.